1. Field of the Invention
The present invention relates to a conductive roller and more particularly to a conductive roller which is used as a developing roller, a cleaning roller, a charging roller, a transfer roller, and the like to be mounted on an electrophotographic apparatus.
2. Description of the Related Art
In the printing technique using an electrophotographic method, improvements have been made for a high-speed printing operation, formation of a high-quality image, colorization, and miniaturization of an image-forming apparatus. Toner holds the key to these improvements. To satisfy the above-described demands, it is necessary to form finely divided toner particles, make the diameters of the toner particles uniform, and make the toner particles spherical. Regarding the technique of forming the finely divided toner particles, toner having a diameter not more than 10 μm and not more than 5 μm have been developed recently. Regarding the technique of making the toner spherical, toner having not less than 99% in its sphericity has been developed. To form the high-quality image, polymerized toner has come to be widely used instead of pulverized toner conventionally used. The polymerized toner allows the reproducibility of dots to be excellent in obtaining digital information as a printed sheet and hence a high-quality printed sheet to be obtained.
In compliance with the improvement in the technique of forming the finely divided toner particles, making the diameters of the toner particles uniform, making the toner particles spherical, and the shift from the pulverized toner to the polymerized toner, a conductive roller which imparts a high extent of charging property to toner and is capable of efficiently transporting the toner to a photosensitive drum is especially useful in an image-forming mechanism of an electrophotographic apparatus such as a laser beam printer, and the like. Users demand that the high-performance function of the conductive roller is maintained to the end of the life of a product.
A conductive rubber roller is proposed, as described in Japanese Patent Application Laid-Open No. 2004-170845 (patent document 1). The conductive roller is composed of the conductive rubber which contains the dielectric loss tangent-adjusting filler for adjusting the dielectric loss tangent thereof to 0.1 to 1.5. The conductive rubber roller is capable of imparting a proper and high extent of charging property to toner, thereby providing a high-quality initial image. In the conductive rubber roller, the charged amount of the toner little decreases even after printing of images on a considerable number of sheets finishes. Consequently the conductive rubber roller keeps providing a high-quality image for a long time.
In the embodiment which is the specific form of the invention, only the epichlorohydrin rubber which is the ionic-conductive rubber is used. Thus toner has an insufficient extent of charging property. Further because the compression set is not less than 5%, the conductive rubber roller is not sufficiently resistant to load.
Such being the case, it is necessary to impart a proper degree of charging property to toner and improve the resistance to load by decreasing the compression set so that the conductive rubber roller can be used for a long period of time.
In the developing roller, when the toner has a low extent of charging property, it is impossible to transport a sufficient amount of toner. On the other hand, when the toner has a high extent of charging property, it is impossible to transport the toner to a photosensitive drum having an opposite electric charge by a static electricity (Coulomb force). Therefore it is important to impart a proper extent of charging property to the toner. In a toner box, the developing roller is rubbed by a flat plate called a developing blade with the developing roller being strongly compressed by the developing blade. Therefore the developing roller is demanded to strain to a possible lowest extent when it is compressed by the developing blade. When a printer is compact because of colorization, the developing roller is demanded to be compact and have a construction which can be compressed to a high extent. Therefore the developing roller is required to have a low degree of strain against the compression.